I think you should've came into contact with CCleaner, which is an awesome tool. It can be set to wipe out directories and dir-trees even, and I added every single Steam-related directory which stores temporary content (caches).

The first thing we going to need (for faster and easier setup) is to set CCleaner to Portable mode. In portable mode, CCleaner will write it's settings to an ini (obliviously ccleaner.ini) rather than to the system registry.

To add external directories for cleaning, we have to use a specified format. (You can also browse every directory one-by-one using the Include tab of options, however, this way is easier.)

IncludeNumber=PATH|path|files(|type)

number is auto-increasing IDpath is the path to be wiped
You can specify only to wipe selected file formats by altering the files variable. Default is *.* which will delete everything.

type is a tricky part. Using the GUI, you have three options how to wipe:

Include files only

Include files and subfolders

Include files, subfolders and the folder itself

Now to specify this, the type values are:

The first option is NULL, you mustn't specify type

|RECURSE

|REMOVESELF

Add these content to the end of the INI.
Please be sure if you remove or add the line, to the IncludeX numbers are properly increasing. Also be sure that you changed D:\Program Files\ for your Steam install directory.

Fire up CCleaner again. (You can check going to Options -> Include to see the directories there.)
On the Cleaner section, in the Windows tree, tick in Advanced\Custom Files and Folders.
Hit Analyze. CCleaner will now search for the custom files (process will consume more time than usual), and after that, click . Steam cached should be wiped from now.

Setting back from portable mode

Go to Options -> Advanced. Untick Save all settings to INI file.

Your registry will be updated with the setting, and the ini will disappear. You're now in installed mode again.

Hope my little tutorial helped saving 500-800 megabytes from your space, and good luck out there! (While writing this article I did an analyze, and CCleaner found 1.6 gigabytes deletable only from Steam caches. And yet I didn't included my Standalone CS:S HLDS directory)